This invention relates to a method and apparatus for retracting the head assembly of a disc drive device and for braking the spindle motor therein once power to the device is interrupted and, more particularly, to utilize the emf that is generated by the spindle motor following power interruption to retract the head assembly and then, after a suitable delay, the motor is dynamically braked.
Disc drive devices have become readily accepted as computer peripheral devices for long term storage of large quantities of data. Such devices typically are constructed as rigid disc drives, also known as Winchester disc drives, and "floppy" disc drives. Rigid, or hard disc drive devices, offer storage capacities that are several orders of magnitude greater than floppy disc drive devices. However, whereas hard disc drives typically contain one, two or more discs that are permanently installed, floppy disc drive devices allow the user to load and unload a floppy disc, thus enabling the user to store different types of information on different discs, as needed. The relative advantages and disadvantages of hard discs and floppy discs are known; and suffice it to say that both types of discs and disc drives have particular uses which, depending upon the application of the user, might favor the use of one over the other.
In hard disc drive devices, the head, or heads (typically, plural surfaces of plural discs are used for the writing and reading of data), "fly" over the surface of the disc. The rotation of the disc creates an air bearing on which the head (or heads) is carried. If this air bearing is reduced, as when the rotating disc slows, the head may crash to the surface of that disc, thus resulting in damage to the disc and to the head and, moreover, resulting in significant loss of recorded information due to the crash. To avoid significant changes in the rotary speed of the disc, the disc drive spindle motor (which rotatably drives the disc) is controlled to detect and account for speed variations. One type of spindle motor control arrangement is described in copending application Ser. No. 706,147, assigned to the assignee of the present invention.
Notwithstanding desired spindle motor speed controls, precautions must be taken to avoid head-disc interference resulting in the destruction of recorded data when the disc drive is deenergized, that is, when power is turned off (i. e. when the usual POWER switch is opened). Therefore, upon opening the POWER switch, circuitry might be triggered, as under microprocessor control, to retract the head (or heads) before power actually is interrupted. That is, the usual head positioning apparatus, such as a stepper motor or a linear head drive motor (both of which are referred to generically as the head actuator) is controlled to move the heads to a "home" position that is safely away from the usable surface of the disc. Once in this home position, there is no danger of destroying useful data when the heads land on the surface of the disc after the termination of rotation following power interruption.
While the aforementioned head retraction arrangement is satisfactory when power deliberately is interrupted, as when the POWER switch is opened, that arrangement might not be called into play in the event of an inadvertent power interruption, such as a power failure. Likewise, this head retraction arrangement might not be initiated during other emergency conditions that result in power interruption. To account for such emergency conditions and power interruptions, it has been proposed heretofore to sense all power interruptions and then retract the heads automatically in response thereto. One such proposal relies upon an inherent time delay between sensed power interruption and significant reduction in disc rotation to generate and supply a "retraction" control signal to the head actuator. However, this proposal runs the danger of a rapid reduction in disc rotation below the critical speed before the head is fully retracted, thus resulting in undesired head-disc interference, or even head crash.
Another head retraction proposal relies upon a large storage capacitor which is charged during normal operation of the disc drive device and, upon sensing power interruption, is discharged through the head actuator. However, such large capacitors occupy a significant amount of physical space within the disc drive device and, in an effort to achieve miniaturization in these devices, such space simply is not available.
Yet another proposal utilizes a relay which, during normal operation of the disc drive device, connects the actuator to the usual head position control circuit. However, upon sensing an emergency condition, the relay is operated to change over the connection of the actuator to the windings of the spindle motor. As this motor continues to rotate following power interruption, an emf is generated in these windings; and this emf is supplied to the actuator in a direction to retract the head. Thus, the "coasting" spindle motor is used to generate an emf which, in turn, powers the actuator for head retraction.
In addition to head retraction upon sensing power interruption, it is desirable to brake the spindle motor, thus preventing coasting of that motor over a significant time period and to impart a "soft", non-destructive landing to the heads. Mechanical braking apparatus requires a substantial amount of space within the disc drive itself and, thus, is not a viable technique for use in miniaturized devices. Although dynamic braking of the spindle motor offers an advantageous alternative, this presents the real danger of head-disc interference when the rotary speed of the disc falls below the critical level needed to support the head. That is, as the spindle motor is dynamically braked, the emf generated thereby falls rapidly before the head actuator is sufficiently driven to retract the head; and the combination of reduced rotary speed and incomplete head retraction will result in the landing of the head onto the usable surface of the disc.